The invention relates to a turbine blade of a turbine, in particular of a gas or steam turbine. The turbine blade extends along a major axis from a root region via a blade leaf region to a head region. The invention relates, furthermore, to a method for producing a turbine blade and to a turbine plant, in particular a gas turbine plant.
The efficiency of a gas turbine plant is determined critically by the turbine inlet temperature of the working medium which is expanded in the gas turbine. The aim, therefore, is to achieve temperatures which are as high as possible. However, because of the high temperatures, the turbine blades are subjected to pronounced thermal load, and, due to the high flow velocity of the working medium or hot gas, to pronounced mechanical load. Blades produced by casting are normally used for the turbine blades. This involves lost-wax casting, partially solidified directionally or drawn as a monocrystal.
A device and a method for production of castings, in particular gas turbine blades, with a directionally solidified structure are described in DE-B 22 42 111. In this case, the turbine blade is cast as a solid-material blade predominantly from nickel alloys in monocrystalline form.
A cooled gas turbine blade may be gathered from U.S. Pat. No. 5,419,039. The turbine blade disclosed in this is likewise produced as a casting or is composed of two castings.
The turbine blades are normally operated at temperatures near to the maximum permissible temperature for the material of the turbine blade, what is known as the load limit. For example, the turbine inlet temperature of gas turbines is approximately 1500 to 1600 K, on account of the temperature limits of the materials used for the turbine blade, and, as a rule, even a cooling of the blade surfaces is carried out. An increase in the turbine inlet temperature requires a larger cooling-air quantity, thereby impairing the efficiency of the gas turbine and consequently also that of an overall plant, in particular a gas and steam turbine plant. The reason for this is that the cooling air is normally extracted from a compressor preceding the gas turbine. This compressed cooling air is therefore no longer available for combustion and for the performance of work. Furthermore, because of the thermal expansion of the turbine blades, it is necessary to have a gap which, above all in the part-load range of the gas turbine, leads to what are known as gap losses.
An object of the invention is, therefore, to specify a turbine blade which has particularly favorable properties in terms of high mechanical resistance and thermal stability. Another object is to specify a method for producing a turbine blade.
These and/or other objects are achieved, according to the invention, by means of a turbine blade which extends along a major axis from a root region via a blade leaf region capable of being acted upon by hot gas to a head region and is formed essentially from carbon-fiber-reinforced carbon, at least the blade leaf region having a blade outer wall with carbon-fiber-reinforced carbon, said blade outer wall being surrounded by a protective layer.
By carbon-fiber-reinforced carbon being used as the material for the turbine blade, the latter has particularly high thermal and mechanical stability. In particular, as compared with conventional monocrystalline turbine blades, higher turbine inlet temperatures up to 2800 K become possible. Preferably, even in the case of large wall thickness differences between the blade leaf region and the solid root region or at the root region and the head region, the same material structure and therefore essentially the same physical properties are achieved in all the blade regions.
By virtue of the particularly high thermal stability of the material used for the turbine blade, it is no longer necessary for the turbine blade to be cooled, with the result that a particularly high efficiency of the turbine plant is achieved. For particularly good oxidation resistance of the carbon-fiber-reinforced carbon, a protective layer is provided, which surrounds at least the blade outer wall acted upon by hot gas when the turbine plant is in operation.
A ceramic layer is expediently provided as a protective layer. In particular, a layer of silicon carbide is suitable for the ceramic layer produced as a straightforward surface layer. The use of silicon carbide has the effect that, by the reaction of the silicon with the carbon, the surface of the turbine blade is sealed with a thin silicon carbide layer and is thereby protected very effectively. On account of its particularly oxidation-inhibiting property, silicon carbide is especially suitable as a protective layer for the turbine blade composed of carbon-fiber-reinforced carbon.
The ceramic layer expediently has a minimum value in terms of its layer thickness of between 0.5 and 5 mm. Depending on the place of installation of the turbine blade, in particular on the thermal load prevailing there, the ceramic layer may also be produced as a multilayer.
In a further particularly advantageous refinement, the protective layer is provided, alternatively or additionally, by a gaseous protective film which is formed by a protective gas. Advantageously, at least in the blade leaf region, a feed for the protective gas is provided, which is surrounded by the blade inner wall. The cavity formed by the blade inner wall makes it possible for the protective gas to be fed in a particularly simple way.
To prevent the oxidation of the carbon-fiber-reinforced carbon, that is to say of the basic material of the turbine blade, natural gas, water vapor or inert gas is advantageously provided as protective gas. For example, exhaust gas, nitrogen or a noble gas is used as inert gas. Use of the protective gas ensures a particularly uniform distribution on the blade surface with the assistance of gas dynamics. The particularly good flow properties of the protective gas thus make it possible to form a closed and surface-covering protective film on the blade surface.
To distribute the protective gas on the surface of the blade outer wall, the turbine blade preferably has a double-shell design at least in the blade leaf region. For example, the wall of the turbine blade may have a double-walled design, with a blade inner wall surrounding the feed and with a blade outer wall extending along the blade inner wall. Between the blade outer wall and the blade inner wall a plurality of cavities are expediently formed which in each case are flow-connected to the feed by at least one associated inlet. In an advantageous refinement, to form the cavities, a plurality of spacers are arranged in the manner of a grid. To reduce the weight of the turbine blade, the spacers are expediently produced from carbon-fiber-reinforced carbon. By the spacers being arranged in the manner of a grid, it becomes possible to have a particularly effective throughflow of the protective gas in the cavities over a long distance. Preferably, in the blade outer wall, a plurality of discharges are provided, which guide the protective gas outward from each cavity. In particular, the feeds and discharges are selected in terms of number and size in such a way that the protective gas flows around the blade outer wall. The protective gas is therefore guided through the turbine blade in an open protective circuit. In this case, the protective gas flows via the discharges, out of the cavities onto the blade outer wall and forms a protective film on that surface of the blade outer wall which is capable of being exposed to the hot gas (comparable to what is known as film cooling). The discharges and the feeds are preferably designed as a bore or a plurality of bores. These may, for example, be widened in a funnel-shaped manner. Such an acute angle is particularly conducive to the formation of a film on the surface of the blade outer wall.
A double-walled construction of this type makes it possible to uncouple the functional properties of the wall structure, while it is possible for the blade outer wall to satisfy lower mechanical stability requirements than the blade inner wall. Consequently, since it is not exposed directly to a hot-gas flow, the blade inner wall can be produced with a larger wall thickness than the blade outer wall and assume essentially the mechanical carrying function for the turbine blade. The cross section of the cavity region between the blade outer wall and the blade inner wall is preferably made as small as possible, in order to generate a high velocity of the protective gas, and, in particular, is in the range of the wall thickness of the blade outer wall. A small throughflow cross section of the cavity and a high velocity of the protective gas thus generated achieve a particularly good protective-film property, especially also an efficient discharge of heat by the protective gas.
The turbine blade is preferably designed as a moving or guide blade of a turbine, in particular of a gas or steam turbine, in which temperatures of well above 1000xc2x0 C. of the hot gas flowing around the turbine blade during operation occur. The blade leaf region of the turbine blade expediently has a height of between 5 cm and 50 cm. The wall thickness of the blade outer wall and/or of the blade inner wall preferably has a minimum value of between 0.5 mm and 5 mm.
Insofar as an object is directed at a method for producing a turbine blade which extends along a major axis from a root region via a blade leaf region to a head region, it is achieved, according to the invention, in that a plurality of carbon fibers are processed in such a way that the carbon fibers form the shape of the turbine blade, there being arranged between the carbon fibers synthetic resin which, when heated under airtightly closed conditions, is converted into a matrix of pure carbon surrounding the carbon fibers.
A turbine blade with sufficient thermal and mechanical strength properties can thereby be produced, which has an essentially identical material structure both in a solid region and in a thin-walled region. The process parameters of the method, for example, winding and adhesive bonding during the processing of the carbon fibers, the temperature and duration of the heating operation and the type of synthetic resin used, etc., are adapted to the size and the desired strength properties of the turbine blade.